1. Field of the Invention
The present invention relates to drill bits for core boring practiced when excavating rock-beds, and more particularly to a drill bit of this kind which has a multi-layered bit head which may be exchanged at the bottom of the bore being drilled.
2. Description of the Prior Art
Core boring has conventionally been practiced for collecting core samples of a stratum when conducting excavation of the same. More specifically, core boring is carried out by rotating a drill bit mounted on the tip end of a boring rod and drilling a bore by means of cutting blades formed at the tip end of the drill bit. However, since the usable life of such cutting blades will come to an end due to wear when drilling proceeds over a certain depth, the blades of the drill bit have to be exchanged during the course of drilling. Unfortunately, the structure of a conventional drill bit is such that it has been necessary when the usable life of the blades ends to pull up the boring rod out of the bore to allow the used blades to be removed and replaced with new ones.
Thus, the usage of a conventional drill bit has involved pulling up the boring rod out of the bore being drilled whenever the operation has required exchange of the blades, thus causing loss of time in raising and lowering the boring rod. The problem has been particularly serious as the depth of a bore drilled increases because the loss of time will increase correspondingly. Therefore, various types of diamond bit for core boring have been proposed which have such structures that the blades can be exchanged at the bottom of the bores without requiring that the boring rods be pulled up out of the bores.
For instance, the present applicant has previously proposed drill bits for core boring as disclosed in Japanese Patent Publications Nos. 60-23591, and 60-129390. A drill bit in accordance with the present applicant's previous proposals comprises: a cylindrical bit body provided with a bit head having cutting blades at its tip portion, and protruding portions at its inner surface; an upper layer bit head including upper layer blades superposed on the cutting blades, rings engaged with the protruding portions on the inner surface of the bit body, and legs connecting between the rings and the upper layer blades; and a disengaging mechanism received in the bit body and operative to disengage the engagement between the protruding portions and the rings, so that the bit heads can be exchanged at the bottom of the bore being drilled.
With the drill bits disclosed in the above Japanese Patent Publications, however, it has been difficult for their structure to simultaneously satisfy the following requirements because these requirements basically run counter to each other: When the upper layer blades are used in drilling, the working upper layer blades should never be displaced relative to the head of the bit body on which the working blades are superposed. On the other hand, when the upper layer blades become worn out and reach the end of their usable life and therefore have to be withdrawn, the used upper layer blades should be capable of being easily removed from their superposed position.